I often receive offers to advertise on my sites most of which I delete because the ads are unrelated, however recently I've received offers that relate to my content. One such offer was $30/month.

The Alexa rankings of my sites are approx 27,000 and 67,000

What criteria do you use to accept advertising on your site?
How do you determine pricing?
Do you charge monthly or yearly?
What type of ads do you accept ie text, banners, etc?